In a multi-hop packet communication network, routes of packets with different sources and destinations may intersect at a common hop, that is, some packets may share a certain link between two nodes in the network. When the combined traffic density on such routes bursts to exceed the link capacity of the common hop, packets queue up at the originating end of the shared link. The queue packets are stored in buffer memory at a node. To preclude buffer overflow for broadband communication applications, some constraint must be exercised over the traffic flow inside the network. Since the transmission capacity of the network has a limit, the packet influx into the network must also be constrained.
The conventional approach to control traffic flow in networks such as the telephone network relies on the use of a control center, which collects and manages real-time traffic data of the whole network. The implementation of such central control incurs huge overhead because of the added requirements of both network facilities and transmission capacity, as well as complexity of design. This renders the approach of having a central control largely ineffective for broadband packet networks.
Communications in real-time, including video, voice, and certain data communications such as computer-to-computer connections, cannot be interrupted once the communication or call is flowing. In order to avoid buffer overflowing, a call set-up process therefore must be responsible for the volume control of periodic streams of packets generated by such continuous real-time communications.
The conventional approach of call set-up is to reserve transmission/switching bandwidths. In the environment of packet switching, this approach is called the circuit emulation mode. In a network with decentralized control, this mode calls for every node to keep track of the total bandwidth of ongoing calls on each of its links and to allocate the link bandwidth for new requests of call set-up. A new call is set up when the bandwidth allocation on all links on its route of transmission is secured. There are some disadvantages with the circuit emulation mode. First, it requires the intelligence of the node to keep track of link bandwidths and to allocate them. Secondly, back and forth inter-node communications is involved in the process of call set-up and tear-down, especially for a mulit-hop call. In order to be compatible with the speed of broadband hardware, it is desirable that the network function of a node requires as little intelligence as possible. This leads to the search of ways of achieving the same objective of circuit emulation without the actual work of bandwidth reservation in setting up a call.